Ah, I've located the law on the Yellow Box - the reason why no one can find it is because it is in subsidiary legislation, which is not easily available to the public.
Anyway here it is:
Causing obstruction to vehicles proceeding along road junction
26. Any person who drives or rides, or turns a vehicle or part thereof into a yellow box indicated by yellow lines connecting the 4 corners of
(a) a road junction and hatching the junction diagonally, notwithstanding that the light signals controlling the movement of vehicular traffic permit him to do so; or
(b) an area of road at a road junction on any part of the road as defined under rule 7 (8) of the Road Traffic (Traffic Signs) Rules (R 33), notwithstanding that he has the right of way,
and by doing so causes obstruction to any other vehicle proceeding to or along that road junction or any part of the road shall be guilty of an offence.
After reading the above, it seems like my earlier view is wrong, and in fact the TP may be correct!
It also means that so long as you don't cause obstruction, you are okay to stay in the yellow box!
What is however, puzzling, is that the term "causing obstruction" can be widely and broadly read; what does it mean?
In such a case, it appears that neither the side road guy nor the main road guy can be in the yellow box, because if any of them is, he would be causing obstruction to the other!
This Rule is drafted so vaguely and widely that I think it has no real meaning
Perplexing to say the least